Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

Abstract

Fire frequency and severity are increasing in high‐latitude regions, but the degree to which groundwater flow impacts the response of permafrost to fire remains poorly understood. Here we use the Anaktuvuk River Fire (Alaska, USA) as an example for simulating groundwater‐permafrost interactions following fire. We identify key thermal and hydrologic parameters controlling permafrost response to fire both with and without groundwater flow, and separate the relative influence of changes to the water and energy balances on active layer thickness. Our results show that mineral soil porosity, which influences the bulk subsurface thermal conductivity, is a key parameter controlling active layer response to fire in both the absence and presence of groundwater flow. However, including groundwater flow in models increases the perceived importance of subsurface hydrologic properties, such as the soil permeability, and decreases the perceived importance of subsurface thermal properties, such as the thermal conductivity of soil solids. Furthermore, we demonstrate that changes to the energy balance (increased soil temperature) drive increased active layer thickness following fire, while changes to the water balance (decreased groundwater recharge) lead to reduced landscape‐scale variability in active layer thickness and groundwater discharge to surface water features such as streams. These results indicate that explicit consideration of groundwater flow is critical to understanding how permafrost environments respond to fire.

Soil Physical, Hydraulic, and Thermal Properties in Interior Alaska, USA: Implications for Hydrologic Response to Thawing Permafrost Conditions

“…Active layer dynamics following wildfire were recently shown to depend critically on organic and mineral soil K fs , relative to other parameters, because of advective permafrost thaw by groundwater flow (Zipper et al, ). This work highlights the presence of measured mineral soil K fs values at nearly 40% (5 out of 13) of the sites, with four out of the five the sites in the rocky classification, in the range of magnitudes shown to have the potential for substantial permafrost thaw via advective heat transport given groundwater flow (Lamontagne‐Hallé et al, ; McKenzie & Voss, ; Sjöberg et al, ; Walvoord et al, ; Zipper et al, ). Detailed and extended site‐based cryohydrogeologic modeling efforts are critical for elucidating the relative importance of advective and conductive thaw of permafrost across the full spectrum of boreal landscapes and ecosystem types.…”

“…Recent cryohydrogeologic modeling efforts have made considerable progress in understanding the most important factors and consequences of changes in permafrost and active layer thickness driven by climate and disturbance shifts. These modeling studies have demonstrated the sensitivity of permafrost table depths to organic layer thickness, vegetation, and snow properties (e.g., Atchley et al, 2016;Briggs et al, 2014;Jafarov et al, 2018;Lamontagne-Hallé et al, 2018;Walvoord et al, 2019); soil thermal properties (e.g., Hinzman et al, 1998); soil saturation (e.g., Chadburn et al, 2015;Rawlins et al, 2013;Subin et al, 2013); soil physical properties such as bulk density and porosity (e.g., Harp et al, 2015;Zipper et al, 2018); soil hydraulic properties such as soil permeability (e.g., Zipper et al, 2018); and parameters such as residual water content (Harp et al, 2015). Recent modeling efforts have simulated unsaturated freeze/thaw dynamics (e.g., Briggs et al, 2014;Lamontagne-Hallé et al, 2018), which requires soil-water retention curve specification.…”

Water Tracks Enhance Water Flow Above Permafrost in Upland Arctic Alaska Hillslopes

“…SUTRA simulates permafrost by incorporating the physics of water phase change, allowing for modeling of pore water freeze and thaw, and permeability variations with changing ice and water content, temperature-dependent variations in liquid water density, and the contribution of the latent heat of fusion to the subsurface energy balance. The freeze-thaw version of SUTRA has been used simulate groundwater flow and heat transport in a number of frozen ground studies (e.g., Briggs et al, 2014Briggs et al, , 2018Evans et al, 2018;Evans & Ge, 2017;Ge et al, 2011;Kurylyk et al, 2014Kurylyk et al, , 2016Lamontagne-Hallé et al, 2018;McKenzie & Voss, 2013;Walvoord et al, 2019;Wellman et al, 2013;Zipper et al, 2018) and has been benchmarked against other coupled cryohydrogeological models by Grenier et al (2018).…”

“…The mineral soil continues to the bottom of the model but remains perpetually frozen below 100 cm, the observed approximate thickness of the active layer. The organic and mineral soil layers were assigned values for permeability, porosity, heat capacity, and solid grain thermal conductivity from previous studies (e.g., Carsel & Parrish, 1988;Hinzman et al, 1991;Jafarov et al, 2013;Kane et al, 1991;McKenzie et al, 2007;Quinton et al, 2008;Treat et al, 2013;Zhang et al, 2010;Zipper et al, 2018). These values are listed in supporting information Table S1.…”

Rapid and Accurate Estimates of Streamflow Depletion Caused by Groundwater Pumping Using Analytical Depletion Functions

“…We used a multistage spin-up to ensure the groundwater and surface water components of our models had reached a dynamic equilibrium prior to beginning our pumping experiments (Somers et al, 2018;Zipper, Lamontagne-Hallé, et al, 2018). First, we ran a steady state simulation with no pumping and recharge rates defined as the long-term average annual baseflow (150 mm/year).…”